Improve the error mail queue

From CONF-3231:

The mail error queue is currently useless.

• You have no indication what the error is
• All you can do is "resend all" or "delete all"

We need to work on the same principles as an MTA.

• If an error is permanent, give up and log the error somewhere.
• If an error is transient, put it on a queue to resend after a few hours.
• If a transient error keeps happening for a certain amount of time, give up and log the error somewhere.

Also useful would be:

• Keep track of users whose mail fails repeatedly, and stop sending them anything. Flag the account, and have a big note on their profile page saying "Notifications are currently disabled for this account due to repeated mail delivery failures. Please turn notifications back on on your notifications preferences page."